Philippe Dollfus

Optimizing the thermoelectric performance of graphene nano-ribbons without degrading the electronic properties

The enhancement of thermoelectric figure of merit ZT requires to either increase the power factor or reduce the phonon conductance, or even both. In graphene, the high phonon thermal conductivity is the main factor limiting the thermoelectric conversion. The common strategy to enhance ZT is therefore to introduce phonon scatterers to suppress the phonon conductance while retaining high electrical conductance and Seebeck coefficient. Although thermoelectric performance is eventually enhanced, all studies based on this strategy show a significant reduction of the electrical conductance. In this study we demonstrate that appropriate sources of disorder, including isotopes and vacancies at lowest electron density positions, can be used as phonon scatterers to reduce the phonon conductance in graphene ribbons without degrading the electrical conductance, particularly in the low-energy region which is the most important range for device operation. By means of atomistic calculations we show that the natural electronic properties of graphene ribbons can be fully preserved while their thermoelectric efficiency is strongly enhanced. For ribbons of width Mu2009=u20095 dimer lines, room-temperature ZT is enhanced from less than 0.26 to more than 2.5. This study is likely to set the milestones of a new generation of nano-devices with dual electronic/thermoelectric functionalities.

Third nearest neighbor parameterized tight binding model for graphene nano-ribbons

The existing tight binding models can very well reproduce the ab initio band structure of a 2D graphene sheet. For graphene nano-ribbons (GNRs), the current sets of tight binding parameters can successfully describe the semi-conducting behavior of all armchair GNRs. However, they are still failing in reproducing accurately the slope of the bands that is directly associated with the group velocity and the effective mass of electrons. In this work, both density functional theory and tight binding calculations were performed and a new set of tight binding parameters up to the third nearest neighbors including overlap terms is introduced. The results obtained with this model offer excellent agreement with the predictions of the density functional theory in most cases of ribbon structures, even in the high-energy region. Moreover, this set can induce electron-hole asymmetry as manifested in results from density functional theory. Relevant outcomes are also achieved for armchair ribbons of various widths as wel...

Transport properties through graphene grain boundaries: strain effects versus lattice symmetry

As most materials available at the macroscopic scale, graphene samples usually appear in a polycrystalline form and thus contain grain boundaries. In the present work, the effect of uniaxial strain on the electronic transport properties through graphene grain boundaries is investigated using atomistic simulations. A systematic picture of transport properties with respect to the strain and lattice symmetry of graphene domains on both sides of the boundary is provided. In particular, it is shown that strain engineering can be used to open a finite transport gap in all graphene systems where the two domains are arranged in different orientations. This gap value is found to depend on the strain magnitude, on the strain direction and on the lattice symmetry of graphene domains. By choosing appropriately the strain direction, a large transport gap of a few hundred meV can be achieved when applying a small strain of only a few percents. For a specific class of graphene grain boundary systems, strain engineering can also be used to reduce the scattering on defects and thus to significantly enhance the conductance. With a large strain-induced gap, these graphene heterostructures are proposed to be promising candidates for highly sensitive strain sensors, flexible electronic devices and p-n junctions with non-linear I-V characteristics.

Non-linear effects and thermoelectric efficiency of quantum dot-based single-electron transistors

By means of advanced numerical simulation, the thermoelectric properties of a Si-quantum dot-based single-electron transistor operating in sequential tunneling regime are investigated in terms of figure of merit, efficiency and power. By taking into account the phonon-induced collisional broadening of energy levels in the quantum dot, both heat and electrical currents are computed in a voltage range beyond the linear response. Using our homemade code consisting in a 3D Poisson-Schrödinger solver and the resolution of the Master equation, the Seebeck coefficient at low bias voltage appears to be material independent and nearly independent on the level broadening, which makes this device promising for metrology applications as a nanoscale standard of Seebeck coefficient. Besides, at higher voltage bias, the non-linear characteristics of the heat current are shown to be related to the multi-level effects. Finally, when considering only the electronic contribution to the thermal conductance, the single-electron transistor operating in generator regime is shown to exhibit very good efficiency at maximum power.

Transport gap in vertical devices made of incommensurately misoriented graphene layers

By means of atomistic tight-binding calculations, we investigate the transport properties of vertical devices made of two incommensurately misoriented graphene layers. For a given transport direction (Ox-axis), we define two classes of rotated graphene lattice distinguished by difference in lattice symmetry and, hence, in Brillouin zone. In particular, these two classes correspond to two different cases where the position of their Dirac cones in the k y -axis is determined differently, i.e. or (L y is the periodic length along the Oy axis). As a consequence, in devices made of two layers of different lattice classes, the misalignment of Dirac cones between the left and right graphene sections opens a finite energy-gap of conductance that can reach a few hundreds of meV. We also show that strain engineering can be used to further enlarge the transport gap and to diminish the sensitivity of the gap on the twist angle and on the commensurateness of the layer stack.

Strain Effects in p-type Devices using Full-Band Monte Carlo Simulations

The particle-based Monte Carlo (MC) technique is acknowledged as a powerful method for accurately describing the carrier transport in semiconductor materials and devices within the semi-classical approximation, i.e. the Boltzmann transport equation (BTE) for the distribution function. It has been developed by many groups to study a wide variety of transport problems in many kinds of devices, to such a point that it is impossible to summarize here the most significant examples of its applications. The accuracy of the semiclassical transport description is then given by the models used for the band structure and the scattering mechanisms. Hole transport properties in realistic Si devices are particularly affected by the strong anisotropy of the valence band (Thomson et al., 2006), which is further increased by the presence of strain. Analytic approximations fail to describe the valence band structure of Si, and an accurate or « full » description of the energy dispersion is then needed to describe hole transport correctly. In this work, the valence band structure is calculated thanks to a stress-dependent 30-band k.p model (Rideau et al., 2006). With this accurate valence band description, « Full-Band » Monte Carlo simulation becomes an appropriate tool to study various (unstrained or strained) p-type devices. Beyond the description of the model in Section 2, we show here two typical examples of application of the full-band Monte simulator to strain effects in p-type devices. In Section 3, the influence of mechanical stress on Double Gate p-MOSFET performance is be investigated. Multiple gate structures are now recognized as promising architectures to overcome short channel effects in nanometer scaled MOSFET. Double Gate MOSFETs (DGMOS) are found to reach the best performance among SOI-based transistors (Saint Martin et al., 2006). In addition, mechanical stress is used as a technological performance booster for CMOS technology. The impact of stress on carrier transport is then of great importance and is both studied experimentally (Huet et al., 2008 ; Suthram et al., 2007) and theoretically (Huet et al., 2008a; Bufler et al., 2008; Pham et al., 2008). In this work, device performance of strained Si p-DGMOS is studied considering uniform biaxial and uniaxial stresses in the channel. The effect of strain is analyzed via some of the main usual figures of

Phonon transmission at Si/Ge and polytypic Ge interfaces using full-band mismatch based models

This paper presents theoretical investigations on the interfacial thermal conductance (Kapitza conductance) in both monotype Si/Ge (cubic 3C) and polytype (cubic 3C/hexagonal 2H) Ge interfaces by using full band extensions of diffusive and acoustic mismatch models. In that aims, phonon dispersions in the full 3D Brillouin zone have been computed via an atomistic adiabatic bond charge model. The effects of crystal orientation are investigated, and the main phonon modes involved in heat transfer are highlighted. According to our calculations, polytype interfaces without any mass mismatch but with a crystallographic phase mismatch exhibit a thermal conductance very close to that of Si/Ge interfaces with a mass mismatch but without any phase mismatch. Besides, the orientations of Ge polytype interface that have been observed experimentally in nanowires, i.e., along [ 115 ] / [ 50 5 ¯ 1 ], exhibit the lowest interfacial conductance and thus may offer new opportunities for nanoscale thermoelectric applications.

Ab initio based calculations of the thermal conductivity at the micron scale

Heat transport in bulk semiconductors is well understood, and during the last few years, it has been shown that it can be computed accurately from ab initio calculations. However, describing heat transport in micro- and nanodevices used in applications remains challenging. In this paper, we propose a method, based on the propagation of wave packets, for solving the phonon Boltzmann transport equation parametrized with ab initio calculations. It allows computing the thermal conductivity of micro- and nano-sized systems, without adjustable parameters, and for any materials. The accuracy and applicability of the method are demonstrated by computing the cross plane thermal conductivity of cubic and hexagonal silicon thin films as a function of their thickness.

Thermoelectric effects in graphene and graphene-based nanostructures using atomistic simulation

Thermoelectric properties of graphene and graphene-based nanostructures have recently attracted great attention from both physics and engineering communities. However, to make graphene a good thermoelectric material, two important issues must be overcome, i.e. (i) its gapless character, which leads to a poor value Seebeck coefficient in pristine graphene and (ii) its high thermal conductivity that leads to low thermoelectric efficiency in graphene devices. By means of atomistic numerical simulation of electron and phonon transport, we show that different techniques of nanostructuring and bandgap engineering can be used to strongly enhance the thermoelectric properties. It includes in particular graphene nanoribbons appropriate shape, hybrid graphene/boron nitride nanoribbbons and graphene nanomeshes and vertical junctions of multilayer graphene. Figures of merit higher than 1 can be obtained at room temperature for such graphene-based nanostructures.

Hybrid states and bandgap in zigzag graphene/BN heterostructures

We study the properties of edge states in in-plane heterostructures made of adjacent zigzag graphene and BN ribbons. While in pure zigzag graphene nanoribbons, gapless edge states are nearly flat and cannot contribute significantly to the conduction, at BN/Graphene interfaces the properties of these states are significantly modified. They are still strongly localized at the zigzag edges of graphene but they exhibit a high group velocity up to 4.3x10^5 m/s at the B/C interface and even 7.4x10^5 m/s at the N-C interface. For a given wave vector the velocities of N/C and B/C hybrid interface states have opposite signs. Additionally, in the case of asymmetric structure BN/Graphene/BN, a bandgap of about 207 meV is open for sub-ribbon widths of 5 nm. These specific properties suggest new ways to engineer and control the transport properties of graphene nanostructures.